The present invention relates to thermosetting silicone resin molding compositions, and more particularly the invention relates to a silicone resin molding composition which is capable of being thermoset at lower temperatures with a reduced time.
Silicone resins have excellent heat resistance, water resistance and electrical properties, and therefore they are now in use in many applications. One of the products obtainable is an insulator for electronic and electrical devices, which is produced by the resin with a filler and catalyst and molding the material into the product.
Since silicone resins used for such molding are generally solid resins that are hard and brittle at room temperature, in order that the resin may be uniformly mixed with a catalyst and filler, it is necessary to heat the resin to a temperature higher than its softening point (50.degree. to 120.degree. C.). During this heating and mixing, if the catalyst acts thus promoting the curing of the resin, this curing tends to impede a smooth molding of the mixed composition. In extreme cases, the curing even tends to make the molding difficult. As a result, the catalyst incorporated in the resin composition must be such that it exhibits no catalytic activity or its catalytic activity is negligible during heating and mixing of the materials but it rapidly activates in the hot fluid state during the molding process.
The catalysts known in the art include lead monoxide, lead oxide and lead carbonate, but these catalysts are disadvantageous in that while, when used singly, these catalysts exhibit weak catalytic activity and do not cause any cure reaction during heating and mixing, the curing during the molding operation cannot be completed within a short period of time. As a means of overcoming these deficiencies, the use of a carboxylic acid, ammonium salt of a carboxylic acid, carboxylic anhydride or the like in combination with such known catalyst has been proposed in U.S. Pat. No. 3,208,961 specification, for example. This method is also disadvantageous in that while the curing can be completed within a short period of time, cure reaction is caused during the heating and mixing thus setting a limit to the available kneading time. Another disadvantage is that a high molding temperature of 170.degree. to 180.degree. C. is usually required, and consequently if the resin is used as an encapsulation material for electronic parts which are susceptible to temperature, there will be a detrimental effect on the encapsulated parts. Still another disadvantage is that since the content of electrolyte impurities in the molding composition is high, the use of the resin on parts for electronic equipment involves difficulty, and moreover the use of a lead compound has the possibility of causing environmental pollution or harm to the human body.